Adsorption kinetics of dyes and their mixtures with Co3O4–ZrO2 composites

• Cobalt oxide networks and zirconia flakes were synthesized by combustion synthesis.
• Co3O4–ZrO2 composites showed good adsorption uptake for MG and CR dyes.
• Activated carbon showed poor adsorption than composites with the mixture of dyes.
• Adsorption kinetics over Co3O4–ZrO2 composites was discussed.

Zirconia–cobalt oxide composites were synthesized by using one step combustion synthesis method. The composites showed different morphologies by varying oxide composition. The combination of porous networks and flakes were obtained with the Co3O4–ZrO2 composites synthesis. The flakes like structures in composites were increased due to increasing the zirconia content. These materials were characterized by XRD, XPS, SEM, TEM and surface area analysis. The adsorption of these composites were examined with both cationic and anionic dyes which include malachite green (MG) and Congo red (CR). 40 percent Co3O4–ZrO2 (40 wt% Co3O4 and 60 wt% ZrO2) has shown high adsorption for MG dye whereas Co3O4 has shown better adsorption capacity for CR dye. The adsorption capacity of Co3O4–ZrO2 was higher than commercial activated carbon for the adsorption of CR and mixture of dyes. This adsorption data was best fitted with pseudo second order (PSO) kinetics. Isotherm models of Langmuir, Freundlich, Langmuir–Freundlich (LF), Dubinin–Radushkevich (DR model) and Temkin were examined on the aqueous MG and CR dye solutions. Selective adsorption of both dyes was also studied with all composites and it was found to be different from the one component aqueous dye solution adsorption. The competitive adsorption was observed with binary mixture of dyes under different pH conditions. The stability of the material was tested and it was found that Co3O4–ZrO2 is stable even after 3 cycles.

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